The importance of PUFAs is undisputed. For example, certain PUFAs are important biological components of healthy cells and are recognized as: “essential” fatty acids that cannot be synthesized de novo in mammals and instead must be obtained either in the diet or derived by further desaturation and elongation of linoleic acid (LA; 18:2 omega-6) or α-linolenic acid (ALA; 18:3 omega-3); constituents of plasma membranes of cells, where they may be found in such forms as phospholipids or triacylglycerols; necessary for proper development (particularly in the developing infant brain) and for tissue formation and repair; and, precursors to several biologically active eicosanoids of importance in mammals (e.g., prostacyclins, eicosanoids, leukotrienes, prostaglandins). Additionally, a high intake of long-chain omega-3 PUFAs produces cardiovascular protective effects (Dyerberg, J. et al., Amer. J. Clin. Nutr., 28:958-966 (1975); Dyerberg, J. et al., Lancet, 2(8081):117-119 (Jul. 15, 1978); Shimokawa, H., World Rev. Nutr. Diet, 88:100-108 (2001); von Schacky, C. and Dyerberg, J., World Rev. Nutr. Diet, 88:90-99 (2001)). And, numerous other studies document wide-ranging health benefits conferred by administration of omega-3 and/or omega-6 PUFAs against a variety of symptoms and diseases (e.g., asthma, psoriasis, eczema, diabetes, cancer).
A variety of different hosts including plants, algae, fungi and yeast are being investigated as means for commercial PUFA production. Genetic engineering has demonstrated that the natural abilities of some hosts (even those natively limited to LA and ALA fatty acid production) can be substantially altered to result in high-level production of various long-chain omega-3/omega-6 PUFAs. Whether this is the result of natural abilities or recombinant technology, production of arachidonic acid (ARA; 20:4 omega-6), eicosapentaenoic acid (EPA; 20:5 omega-3) and docosahexaenoic acid (DHA; 22:6 omega-3) may all require expression of a delta-5 desaturase.
Most delta-5 desaturase enzymes identified so far have the primary ability to convert dihomo-gamma-linolenic acid (DGLA; 20:3 omega-6) to ARA, with secondary activity in converting eicosatetraenoic acid (ETA; 20:4 omega-3) to EPA (where DHA is subsequently synthesized from EPA following reaction with an additional C20/22 elongase and a delta-4 desaturase). The delta-5 desaturase has a role in both the delta-6 desaturase/delta-6 elongase pathway (which is predominantly found in algae, mosses, fungi, nematodes and humans and which is characterized by the production of gamma-linolenic acid (GLA; 18:3 omega-6) and/or stearidonic acid (STA; 18:4 omega-3)) and the delta-9 elongase/delta-8 desaturase pathway (which operates in some organisms, such as euglenoid species and which is characterized by the production of eicosadienoic acid (EDA; 20:2 omega-6) and/or eicosatrienoic acid (ETrA; 20:3 omega-3)) (FIG. 1).
Based on the role delta-5 desaturase enzymes play in the synthesis of e.g., ARA, EPA and DHA, there has been considerable effort to identify and characterize these enzymes from various sources. As such, numerous delta-5 desaturases have been disclosed in both the open literature (e.g., GenBank Accession No. AF199596, No. AF226273, No. AF320509, No. AB072976, No. AF489588, No. AJ510244, No. AF419297, No. AF07879, No. AF067654 and No. AB022097) and the patent literature (e.g., U.S. Pat. Nos. 5,972,664 and 6,075,183). Also, commonly owned, co-pending application having Provisional Application No. 60/801,119 (filed May 17, 2006) discloses amino acid and nucleic acid sequences for a delta-5 desaturase enzyme from Peridium sp. CCMP626, while commonly owned, co-pending application having Provisional Application No. 60/915,733 (BB1614) (filed May 3, 2007) discloses amino acid and nucleic acid sequences for a delta-5 desaturase enzyme from Euglena anabaena. 
The instant invention concerns the identification and isolation of additional genes encoding delta-5 desaturases from Euglena gracilis that would be suitable for heterologous expression in a variety of host organisms for use in the production of omega-3/omega-6 fatty acids.